Spatial variability of nitrate concentration in lakes in Snowdonia, North Wales, UK

Abstract. At a regional scale, high nitrate (NO₃^‾) concentrations in upland surface waters generally occur in tandem with high nitrogen (N) deposition levels. However, significant differences in the patterns of surface water NO₃^‾ concentration have been observed within areas of similar N deposition yet relatively few studies have been undertaken which examine within-region variation of NO₃^‾ concentrations. A study of 76 lakes in Snowdonia, north Wales, an area of high deposition and sensitive catchments, was undertaken to assess variation in surface water NO₃^‾ concentration across a 20 x 20 km grid square and to identify catchments vulnerable to NO₃^‾ leaching. Nitrate concentrations vary considerably, particularly during winter when values range from 0.7 to 70 μeq l^-1. Although retention by vegetation and soil microbes in summer reduces the amount of NO₃^‾ reaching the lakes, 37 % of sites are characterised by NO₃^‾ concentrations greater then 4 μeq l^-1. The elevated concentrations occurring in summer suggests that N breakthrough has occurred. By examining the ratio of NO₃^‾ to total strong acid anions, it is shown that NO₃^‾ contributes significantly to freshwater acidity, particularly during the winter. Redundancy analysis shows that NO₃^‾ leaching is greatest, both in winter and summer, in catchments with high proportions of bare rock where soil and vegetation cover is limited. Nitrogen cycling in these catchments, generally at higher altitudes, may no longer be governed by seasonal biological controls. They are likely to be saturated with respect to nitrogen (i.e. incapable of further N retention) as a result of the elevated deposition levels in the area. The contribution of NO₃^‾ to acidity is also greatest at these sites. In winter, significant positive relationships are also evident between NO₃^‾ concentration and soil pH and coniferous woodland. The study demonstrates the importance of catchment factors in modifying the relationship between N deposition and N leaching in upland catchments.